The hashcat core was completely refactored to be a MT-safe library (libhashcat).
The goal was to help developers include hashcat into distributed clients or GUI frontends.
The CLI (hashcat.bin or hashcat.exe) works as before but from a technical perspective it's a library frontend.

There's also new features, new hash-modes, many bugfixes and performance improvements.

I recommend upgrading even if you did not face any errors with older versions.